A top climate scientist isn’t impressed with the world’s half-assed effort to save itself.

According to a paper released Tuesday by James Hansen, formerly of NASA and now at Columbia University*, the landmark Paris Agreement is solid C-minus work — but when it comes to climate commitments, mediocrity is criminal. Slacker countries making only modest emissions reductions will lock future generations into dangerous levels of climate change.

The average global temperature is already 1 to 1.3 degrees Celsius warmer than preindustrial levels, according to Hansen’s group. That’s on par with the Earth’s climate 115,000 years ago, when the seas were 20 feet higher than they are today.

Unless we phase out fossil fuels entirely in the next few years, Hansen told reporters on Monday, future generations will have to achieve “negative emissions” by actively removing carbon from the atmosphere. Seeing as we don’t even know if that’s possible, that’d be a helluva task for our progeny.